Hob-cutter.



' l l W. VF. ZIMMERMANN.

HOB GUTTER.

MPL10 IIIIIII LED FEB. 111111 7.

1,003,024. Patented sept. 12,1911;

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/1770HNEV5- W. P. ZIMMERMANN.

110B GUTTER.

APPLICATION FILED FEB. 18, 1907.

1,003,024. Patented 12,1911.

2 SHEET 2.

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WILLIAM F. ZIMMERMANN, F NEWARK, NEW JERSEY.

HOB-CUTTER.

. specification or Letters' Patent.

Patented Sept. 12, 1.911.

l Application led February 1'8, 1907. Serial No. 357,844.

To a/Zl whom it may concern:

Be it known that I, WrLLrAM F. ZIMMER- MANN, a citizen of the UnitedStates, and a resident of the city of Newark, county of Essex, andStat-e of New Jersey, have invented certain new and useful Improvementsin Hob-Cutters; and I do declare the following to a full, clear, andeXact description of my invention, such as will enable others skilled inthe art to which it pertains or to which it is most nearly connected tomake, construct, and use the same.

Figure 1 is a side elevation of a helical cutter embodying theinvention, with the cutting edges in a plane normal to the helices. Flg.2 is a similar elevation, but with the cutting edges in a plane parallelto the axes of the cutter. Fig. 3 is an enlarged cross section of theteeth of the cutters on the line af: of Fig. 1 or on the line g/ y ofFig. 2. Fig. 4 is a sectional view illustrating a helical cutter withconcave cutting edges in connection with a series of resulting gearteeth, and also showing a dotted section of a theoretically correctcutter and the teeth resulting therefrom. Fig. 5 is a similar viewillustrating a helical cutter with concave cutting edges, together witha shortened pitch, and also illustrating the diference between atheoretical tooth curve and the resultant toth curve obtained by themodified cutter.

It is well known that gears when cut theoretically correct with astraight side rack tool, produce an objectionable humming noise at highspeed. This may be explained in the following manner. When the teeth ofwheels are correctly constructed, that is, in accordance with the theoryof gear cutting, so as to produce absolutely continuous motion, it may'be readilyshown that when the point of contact between two .intermeshingteeth is substantially in the line of centers, the adjacent teeth areout of contact and as the two contacting teeth advance, there is asensible space of time when there is a single pair of teeth in Contactand another sensible space of time when two pairs are in contact. Whenthe first pair recedes and the second pair approaches, it leaves onlythe second pair in contact, thus throwing the entire load upon thatpair, until the next pair approaches in contact when .the strains areagain divided between the two pairs. These sudden kchanges of strainsfrom full load to half load cause a series of shocks, which at highspeed, produceV the humming noise heretofore referred to. I havediscovered that by deviating slightly from the theoretically correctrack tooth, as for instance, curving the` cutting edges or shorteningthe pitch or both, am able to completely overcome this h ummlng noiseand produce practically sllent running spur gears. The result of thismodification to the theoretical rack tooth is such that the points ofthe gear teeth are slightly relieved and the bases under cut, therebyforming a'curve which w1ll be of such a character that when two matinggears have their teeth in contact, the load of the receding pair ofteeth is gradually diminished and simultaneously the pressure on theapproaching pair is gradually increasing so that there is no suddentransit-ion from full load to half load and from half load to full loadbeis a tooth formed similar to a truncated l wedge with straight sides,and is the base of the involute system of gear tooth curves. Thetheoretical pitch of a rack tooth is the distance between the center oftwo consecutive teeth measured on the pitch line. In measuring the pitchof a rack tooth, the same is always measured on a straight line, whereasthe pitch of a gear is measured on a curved line. A rack, however, ismerely a gear with an innite radius. The pitch of a gear is determinedand fixed by the number of teeth in the pitch circle of the gear. Tofind the pitch of a gear, the product of the pitch circle by the factor3.14159 is divided by the number of teeth in the gear. The pitch thusfound would be known as the circular pitch of the gear and is equal tothe arc subtended at the pitch line by the angle formed at the center inbisecting two adjacent teeth. This circular pitch rack when rectiied,would be the pitch of the rack teeth and would then be known as thelinear pitch of the rack.

In Fig. l I have shown a hob cutter for use in cutting the teeth ot'gear wheels in which the slots a. are normal to thel helix. Asillustrated in Fig. 3, which shows a crosssection'ot' the teeth, it willbe noted that the contour ot' the tooth is symmetrical with respect toits axis I). This style of cutter may be formed either upon a lathe orupon a milling machine. The usual method of iorming a hob cutter upon alathe results in a tooth upon the cutter which is not symmetrical in theplane normal to the helix and this for the reason that the cutting edgesof the tool used in cutting the hob are made symmetrical in a planepassing through the axis of the hob; or, in other words, the

cutting edges of a lathe tool are symmetrical in the plane of itsmovement when forming a hob. This results in producin a cut-ter withteeth which are not symn'ietrlcal in the plane normal to the helix,softhat when in use with this normal plane parallel to the face of thegear to be cut, the. sides ot' the,

teeth do not conform to the correct shape of the tooth to be cut in thespur gear Wheel,

resulting in producing unsymmetrical sides in the teeth of the finishedblank. The method which I have just described, is the ordinary method ofproducing hobs of this character. I obviate this difficulty by placingthe. tool with which the helical cutter is formed, with the plane of itscutting edge at an angle with the axis of the cutter, equal to the angleof the helix, or in other words, the plane of the cutting edge of thetool lies either in the plane normal to the helix or in a. planeparallel tothis normal. It will thus be noted that the normal pitch c ofthe helix upon the hob cutter so formed is less thanthe usual linearpitch given to cut-ters of this type. In using a cutter formed as l.have described to cut' spur gears, it wil be found that when set at. anangle equal yy/tlie angle et the helix, the shapes of the/teeth will notbe foreshortened by reason "of the unsymmetrical sides, as would be.the, case if the tool used in cutting the hbb" were placed in a planeparallel with the axis of the hob. When a hob of this character is cutupon a lathe, such lathe must also be geared up for a longer lead thanthat which is given by calculation across the normal of the helix, sothat when placing the cutter at an angle as stated above in cutting spurgears, the actual lead will be at right angles to the face of the spurgear bein eut; or in other Words, the gearing up ol the lathe must beequal to the linear pitch of t-he helical cutter and net to the normalpitch. If the helical or hob cutter is fortned in a mill-v ing machine,all of these points are taken care of, because of the method ofproducing the cutter, for under such circumstances the milling cutter isplaced at an angle with the axis of the cutter blank, equal to the angleof the helix. In using a cutter of this character to cut spur gears, asI have stated above, it is necessary to set the axis of the 'cutter inthe cutter carriage at an angle'with the horizontal equal to the angleof normal to the helix. Under certain conditions this is objectionablefor the reason that the machine must be accurately set, and such anglebe accurately gaged for each cutter.

In Fig. 2, I have shown a cutter having its teeth so shaped that it isnot necessary in cutting spur gears to set the axis of the cutter off atan angle equal to the angle of the helix of the cutter. This cutter isformed in a lathe by a tool the shape of whose cutting edge issymmetrical with respect to its axis and placed parallel with and movingparallel to the axis of the cutter, thereby producing a tooth which issymmetrical in the diametrical plane through the cutter and one in whichthe actual lead is also correct and in a plane parallel to the axis ofthe cutter, so that when used as it has been formed, it will producesymmetrical teeth in the blank to be eilt. It has also the advantage ofeasing off or relieving the points and bases of the teeth, because inboth of these cutters just described, the sides d of the teeth are notformed of truncated wed es, but they are formed of reverse curves ansliertened pitch so thatthe sides d will relieve the teeth at the.points as well as undercut them at the base, and thereby give thedesired result of producin practicall noiseless gears', that 1s, (fears1n which w en two intermeshing teethlliave their points of contact intheir line of centers, the advancing and receding teeth will also be inslight contact and at this point, the strain exerted between the twogears will be divided in substantially the proportion-of one half uponthe two teeth in contact in the line of centers while the advancing andreceding teeth will carry one uarter of the load. As the teeth advance,t e pressure upon the receding teeth will gradually increase, butin asomewhat greater ratio, inasmuch as the teeth which have just been inline of centers are also now the receding pair and the pressure uponthem is also gradually diminishin Re erring now particularly to Fig. 4,a dotted section of rack teeth is drawn to represent a theoreticallycorrect section of rack based on the accepted standard formula. Thesection of rack shown in full lines is one which has been modified inaccordance with the invention. It should be noted that the differencebetween a theoretical rack and a modified rack is in that the cuttin edes or sides of the teeth of the modifie racz are concave with respect totheir axes, whereas in the theoretical rack the sides are erfectlystraight. The difference in resuIt when using a hob cutter with curvedcutting edges over one With straight cutting edges, can be readily seenfrom the resulting gear teeth shown, in Which the dotted gear teethsection represents those produced by the theoretical rack and the gearteeth drawn in full lines, represent those produced by a rack having itscutting edges modified. The difference between these two results isobvious and resides in the fact that the points of the gear teeth areslightly rounded, gradually 'increasing in amount from the pitch lineout. The bases are also undercut, gradually increasing in amount fromthe pitch line in.

Referring now to Fig. 5, in this figure the dotted section of rack teethalso represent a theoretical rack, and the dotted gear tooth curves, theresultant gear teeth. The rack teeth in full line have their cuttingedges curved symmetrically with respect to their axes, and in additionhave the normal circular pitch o of less length than the theoreticalpitch. By inspection of the drawing, it Will be seen that by shorteningthe itch and curving the cutting edges of the t eoretical rack teeth,the points of the resulting gear teeth are relieved, and the basesundercut similar to that shown in Fig. 4, but with this exception, theshortening of the pitch of the rack teeth relieves the points of theteeth slightly more in proportion to the under cutting of the bases.This means that to ob tain the same quietness in the running of theresultant gear 'teeth as that obtained by the cutter With curved edgesonly, as shown in Fig. 4, the cutting edges in Fig. 5 would not have tobe curved quite so -much because by shortening the pitch, the pointsWould be relieved to a greater extent than the bases are undercut, andwhich would result in stronger teeth.

. Claims.

l. A helical or hob cutter for generating gear teeth, having the cuttingedges in a helical plane and' symmetrical With their axes, said cuttingedges curved to a degree differing from a theoretical straight sidedrack tooth.

2. A helical or hob cutter for generating gear teeth, having the cuttingedges in a helical plane and concaved with respect to their axes to adegree differing from a theoretical straight sided rack tooth.

3. A .helical or hob cutter for generating gear teeth, having thecutting faces in a helical plane and symmetrical vWith their axes, saidcutting faces concaved to a' degree i length and of symmetrical contourin respect to their axes in a plane normal to the helices of saidcutter, and the normal pitch of the helices of less length than thepitch of a theoretical straight sided rack tooth.

6. A helical or hob cutter With concaved cutting faces, symmetrical withrespect to their axes in a plane normal to the helices of the cutter.

7. A helical 0r hob cutter, the cutting faces of which are in a helicalplane and symmetrically concaved With respect to their axes in a planenormal to the helices, and the pitch of said helices slightly shorterthan the pitch of a theoretical straight sided rack fOOth. z

This specification signed and witnessed Witnesses: p

LOUIS M. SANDERS, C. A. ALLrsToN.

